Mysterious Vibration Cured

Thanks to help found on this site, I was able to eliminate a severe vibration.

Background: Merc 850 Big Four power head on a Mark 55/ Merc 400 type tower. The swap (to build a far lighter motor) entailed adapting the P H and different drive shaft splines. To my horror a severe vibration cropped up at about 4,000 rpms, a vibration that shook the entire boat! One response insisted similar PHs had been adapted to racing LUs without a problem--and that put me on the right track.

The photo below shows the original and modified (11 spline to 8 spline) drive shaft adapter. That extension welded to the adapter eliminated a wobble caused by slop in the splines, a wobble that allowed destructive sideways motion. The extension is tightly fitted to the drive shaft via aluminum foil tape (since the splined area is larger in diameter than the shaft below it).

With my test prop I was able to wing her up to 4,400 rpms and beyond without a vibration issue. Whooppee!

Time to build an even wilder "Diesel" (as I call Merc 850s with its super mild porting).

Jeff

PS: Once again, thanks for all the great help! I was a lost ball in the tall weeds with a potentially excellent set up that was unusable due to vibration issues.